You could use the concat() function to store an array.
If your listbox field was [FieldName] then your variable definition would be:
concat( distinct [FieldName], ' ;' )
It will create a long string delimitted by the 2nd arguement in this example a ';' .
You could check the string for the presence of a specific string using substringcount(). If the variable was vSelectedValues this would be a sample usage. Use $ sign expansion to invoke the variable like this:
IE: if( substring( $(vSelectedValues) , 'Sample Value') > 0 , red(), blue())